Correct bug of not deleted link

[DaSilvaRemi]

I correct the BUG of the Link doesn't delete when it's not connected to a target port related on issue #895

Checklist

• The code has been run through pretty yarn run pretty
• The tests pass on CircleCI
• You have referenced the issue(s) or other PR(s) this fixes/relates-to
• The PR Template has been filled out (see below)
• Had a beer/coffee because you are awesome

What?

Correct the bug linkModel.ts because link doesn't delete when you delete a link by it point.

Why?

because the link doesn't delete

How?

delete the property to avoir NULL value

Feel good image:

[DaSilvaRemi]

Pls accept my pull request

[DaSilvaRemi]

@dylanvorster